Kinetics and mechanism of an adsorptive displacement on polyoxotungstate modified electrodes

An electrochemical approach has been used to switch adsorbate monolayers on a mercury surface between two competing adsorbates. The principle is that the adsorption strength of silicotungstic acid (SiW$\sb{12}$) is stronger than that of the oxidized form of 2,6-disulfonated anthraquinone (2,6-AQDS) but weaker than that of the reduced form of 2,6-AQDS. Thus, the identity of the molecules adsorbed on the electrode surface in contact with a solution of 2,6-AQDS and SiW$\sb{12}$ can be selected conveniently by the redox state of 2,6-AQDS which, in turn, is controlled by the electrode potential.This unique system constitutes a model system for the study of intrinsic adsorption displacement kinetics and mechanism. In the fully reduced state of 2,6-AQDS, there is preferential adsorption of 2,6-AQDS over SiW$\sb{12}$. On switching the electrode potentials from fully reduced to fully oxidized states of 2,6-AQDS, the reduced 2,6-AQDS molecules are reoxidized and then displaced (desorbed) from the surface by SiW$\sb{12}$ molecules. Linear sweep voltammetry was used to probe the amount of 2,6-AQDS remaining on the surface. The displacement rate of 2,6-AQDS was obtained from repetitive probing at different times following the potential switching. The experimental results shows that the displacement rate follows first order kinetics with respect to dissolved 2,6-AQDS and second order kinetics with respect to dissolved SiW$\sb{12}$. The rate equation suggests two parallel displacement mechanisms: sequential desorption followed by adsorption, analogous to S$\sb{\rm N}$1, and concerted adsorption and desorption, analogous to S$\sb{\rm N}$2.The basic electrochemical behaviors of SiW$\sb{12}$ both in solution and on surface is described in detail. They are discussed in two separate chapters.The thesis also discusses the effect of anodic dissolution of a Pt counter electrode on the electrochemical performance of the working electrode. In an acidic solution, the application of a sufficiently negative potential on a carbon working electrode can induce a very positive potential at which the Pt counter electrode corrodes. The dissolved Pt species (oxo-species) can be transported to and thus deposit on the working electrode. The deposited Pt metal is highly dispersed and accounts for the high electrocatalytic activity of this "contaminated" carbon surface.U of I OnlyETDs are only available to UIUC Users without author permissio